This invention relates to an automatic gain control circuit which is provided in a demodulation section of a modem (modulator-demodulator) to adjust the loop gain so that the level of a band-limited receive signal may have a predetermined reference value and supplies the output signal to an automatic equalization section.
The transmission speed has exhibited a remarkable increase in recent years, and as the increase of the transmission speed progresses, it becomes increasingly necessary to reduce the occurrence of errors in data communication arising from disturbances of the circuit to as few as possible. Further, when an error in data communication actually occurs, a modem is required to follow up the condition of the circuit in a short time.
FIG. 7 shows a general construction of an on-line system. Referring to FIG. 7, in the on-line system shown, a plurality of modems 203 are connected to a host computer 201 by way of a communication control apparatus (CCP) 202, and each of the modems 203 is connected by way of an analog circuit 204 to another modem 203' installed at another location. A terminal 205 is connected to each of the modems 203'.
The on-line system further includes a network supervisory apparatus 206, for which a secondary channel is used.
By the way, a state signal of a modem can be transmitted from each of the host side modems 203 shown in FIG. 7, as it is to the network supervisory apparatus 206. But from each of the terminal side modems 203', a state signal thereof is transmitted to the associated host side modem 203 so that it is transmitted by way of the modem 203 to the network supervisory apparatus 206.
Since a state signal of a modem must necessarily be transmitted without having any influence on main data, each of the modems 203 and 203' divides, for example, a voice band of 0.3 kHz to 3.4 kHz by frequency division to provide a secondary channel for secondary data in addition to a main channel for main data as seen in FIG. 8.
It is to be noted that phase shift keying (PSK), orthogonal amplitude modulation (QAM) or some other modulation is used for a main signal while frequency shift keying (FSK) is used for a secondary signal.
Each of the modems includes, in its reception section, in addition to a demodulation section, a roll-off filter, an automatic gain control section (AGC), an automatic equalization section (AEQ), a carrier phase correction section (CAPC), a timing extraction section, a carrier detection section and so forth. Particularly, the automatic gain control section functions as an automatic gain control circuit for adjusting the loop gain so that the level of a band-limited demodulation signal may be equal to a predetermined reference value, and for inputting the demodulation signal to the automatic equalization section. The automatic gain control section is necessary in order to cause the automatic equalization section to operate regularly.
Such modems are constructed such that, once a disturbance of the circuit such as a gain hit or a drop-out occurs to cause a data error, they restore their normal state through a data mode leading-in operation.
With such modems, however, once a disturbance of the circuit such as a gain hit or a drop-out occurs as described above, they sometimes enter into a data mode leading-in operation. Thus, there is a concern in that, once such data mode leading-in operation is entered, no communication can be performed for the period of time of the operation.
In this instance, the period of time does not matter with a modem of the low speed type since the time required for recovery after starting of a data mode leading-in operation is short, but it cannot be ignored with a modem of the high speed type since the time required for recovery is long.
Also, a method of decreasing the time required for recovery after starting of a data mode leading-in operation is available, however, a large scale is required for a hardware or software construction for realizing the method.